Flush mounted acoustic sensor

ABSTRACT

This invention is a dynamic pressure sensor of the flush mounted variety for use in sensing the presence of an acoustic energy source in a moving fluid environment. The sensor is especially adaptable to moving vehicle applications wherein the sensor is located on the moving vehicle and boundary layer interference due to movement of the vehicle and the sensor in air is a prime consideration.

United States Patent Horwath [54] FLUSH MOUNTED ACOUSTIC SENSOR [72] Inventor: 'fibor G. Horwath, Alexandria, Va.

[73] Assignee: The United States of America as represented bytheSecretar-yof the Army [22] Filed: Oct. 23, 1970 [21] Appl.No.: 83,561

[52] US. Cl. ..'.340/15, l81/0.5 R, 181/0.5 VM, 340/16 R [51] Int. Cl. H04b 11/00 [58] Field of Search ..l81/0.5 R, 0.5 VM; 340/8 R, 340/15,16R,3T,7R

[56] References Cited I UNITED STATES PATENTS 3,485,318 12/1969 Eichler ..l81/0.5 R

[45] July 18, 1972 3,500,260 3/1970 White ..181/0.5 R X Primary Examiner-Richard A. Farley Attorney--Harry M. Saragovitz, Edward J. Kelly, Herbert Berl and Glenn S. Ovrevik [57] ABSTRACT This invention is a dynamic pressure sensor of the flush mounted variety for use in sensing the presence of an acoustic energy source in a moving fluid environment. The sensor is especially adaptable to moving vehicle applications wherein the sensor is located on the moving vehicle and boundary layer interference due to movement of the vehicle and the sensor in air is a prime consideration.

3 Clains, 2 Drawing Figures lNFIIIPISE ERFEREN SIGNAL CE Patented July 18, 1972 2 Sheets-Sheet l INVENTOR HORWATH TIBOR G.

ATTORNEYS AGENT-- Patented July 18, 1972 2 Sheets-She FIG. 2

AGENT- M )3.

232:: DIFFERENCE W 0 ISON MEANS @9- 3e 37 I 5| 1|: F] I FREQUENCY n/ 52 VCO DISCRIMINATOR DIFFERENCE 394 SiGNAL W 36 37 I nwENroR TIBOR G HQRWATH I BY 772- v W w ATTORNEYS FLUSH MOUNTED ACOUSTIC SENSOR GOVERNMENT USE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to me of any royalties thereon.

BACKGROUND OF THE INVENTION A wide variety of detectors have been employed in homing systems for seeking and acquisition of a moving target. In general, these homing systems employ detectors sensitive to one or more of the various types of signatures emitted by the moving target. It is well known, for example, that temperature sensitive sensors have been employed to track the path of sub- 7 marines operating under water long after their departure from the area. Likewise, infrared detectors have been employed as homing means to guide missiles to the exhaust of jet aircraft in flight.

It will be appreciated that all powered moving targets emit an acoustic signature due to motion of the target as well as power plant noise. Heretofore, acoustic signature sensing systems have been largely restricted to fixed position detectors in quiet locations. It will be appreciated that homing systems necessitate installation of the sensing means on a moving vehicle and this in turn introduces boundary layer interference noise due to movement of the vehicle and the sensor in air. Such interference noise must be minimized, of course, if acoustic signature detectors are to be utilized efiectively. Generally, flow noise reduction techniques, applicable to flush mounted acoustic detectors, substantially reduce the signal to noise ratio of the detectors. No prior art flush mounted acoustic detector which affords the essential noise reduction and is sufficiently sensitive for use in sophisticated acoustic homing system is presently known.

SUNINIARY OF THE INVENTION to guide the vehicle toward the target. The motion of the sensor relative to air creates, on the surface of the sensor, a boundary layer of potentially unstable flow which causes pressure fluctuations to produce a second input signal. Likewise, the surfaces of the vehicle moving in air experience boundary layer pressure fluctuations which radiate toward the acoustic sensor to produce a third input signal.

It will be appreciated that the basic objective in the design of an acoustic sensor is to enhance the signal to noise ratio of dynamic pressures, that is, to minimize the interference effect of the second and third input signals on the target signal.

The sensor of the present invention embodies a relatively large sensitive surface area which enables a direct summation of local boundary layer pressures. The large sensitive area of the sensor of the present invention consists of a porous, effectively flat, surface. The sensor, which may be termed a porous surface sensor, is designed to receive the three principal input signals through its porous wall. A uniform frequency response is achieved when acoustic waves inside the sensor acoustic chamber propagating toward the front of the chamber are absorbed. Optimum performance, maximum sensitivity, and signal to noise ratio, is obtained when the reflection coefiicient of the waves propagating toward the microphone end of the chamber is unity independent of frequency.

The uniform frequency response and the maximum sensitivity of the sensor are achieved by the introduction of an internally disposed reflective surface of selected configuration.

The disclosed novel sensor structure effectively minimizes the noise interference of the second and third input signals on the target signal. In addition, this structural configuration provides a flush mounted acoustic sensor wherein magnitude of the sensed signal is indicative of the angle of incidence of the signal.

These features and other significant objects of the invention will become apparent from a clear understanding of the invention for which reference is had to the description of a preferred embodiment of the invention and the drawings wherein:

FIG. 1 is cross section view of the flush mounted sensor of this invention showing the internal structure thereof in a preferred embodiment, together with its associated microphone.

FIG. 2 pictorally depicts an array of flush mounted sensors in a typical orientation with a typical signal processor connected to the sensors for obtaining guidance information.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The sensor shown for illustrative purposes in FIG. 1 comprises an elongated, wedge shaped acoustic chamber 11 of selected length L and width W with one wall, the sensitive surface 12, flush mounted to minimize pressure fluctuation along the surface of the sensor as the carrying vehicle moves through the air. A microphone 13 is connected to one end of the elongated acoustic chamber 11 by means of an acoustical conduit 14. It will be appreciated that the acoustical conduit 14 may be omitted, if desired. For reasons which will be discussed in more detail hereinafter, a wave energy reflective surface 15 is disposed within the acoustic chamber 11 and ex tends substantially the entire length of the acoustic chamber 1 1.

In accordance with the preferred embodiment of the invention, the sensitive surface 12 of the acoustic chamber 11 is made of sintered stainless steel particles of selected size and the sensitive surface 12 is identified by its manufacturer, Mott Metallurgical Corporation, Farmington, Connecticut, 06032, in accordance with micron size of the sintered stainless steel particles. This particular thin wall product is more commonly used in air filtration application and has been found to have a substantially uniform porosity as measured by its flow resistance characteristic.

(p/ Where p pressure drop across the sample Newtonslm u volume velocity of airflow across the sample (m /sec) F =flow resistance (MKS acoustic ohms) For purposes of this invention, flow resistance F, is measured at low values of pressure drops Ap across the porous wall, i.e., in the region Ap== 0.1 1.0 inch water.

It has been found that for acoustic waves at low intensities, the flow resistance F' is constant independent of amplitude. In a typical flush mounted sensor, the sensitive surface 12 could be one-sixteenth inch in thickness, 12 inches in length, and 1.5 inches in width, and made of 2 micron material having a flow resistance of approximately pc, measured at low values of pressure differential across the porous wall where p is the density of the fluid medium and c is the velocity of sound in the fluid medium. Where air is the fluid medium,

p (density of the fluid medium) 1,293 kgm' c= (velocity of sound in the fluid medium) 3,34. 10 in sec The sintered stainless steel particle material disclosed as the sensitive side wall surface herein may be physically described as a relatively thin material made of metallic particles of micron size and having a multitude of randomly distributed and substantially closely spaced, finite, nondescript channels which determine the flow resistance characteristic of the material in selected fluid mediums.

While the acoustic chamber 11 is shown as rectangular with a linear graduation in height in the illustrated embodiment with a sensitive surface 12 of uniform thickness, it will be appreciated that other elongated configurations of semicircular or trapezoidal cross section, for example, may be utilized in selected flush mounted sensor applications with the outer configuration dependent upon aerodynamic considerations and the inner configuration largely dependent upon standing wave considerations.

Furthermore, it will be appreciated that it is within the purview of this disclosure to substitute other porous sensitive surface wall materials having a suitable flow resistance characteristic and to adapt same, if necessary, to support wave energy within the acoustic chamber 11. However, it should be understood that nonporous wall surfaces which have been perforated to provide a selected flow resistance are not interchangeable with porous wall surfaces in this invention. In particular, it has been found that for a given size sensor, the maximum flow impedance obtainable, which would be otherwise suitable for use, is several orders of magnitude too low for use in this invention. Moreover, perforations afford an irregular surface which creates undesireable turbulence at the sensitive surface and have been found to generate a side lobe interference factor which, dependent upon size of the perforations and spacing therebetween, may create a noise source of considerable consequence... a a 7 As shown in FIG. 1, the microphone 13 may be connected to one end of the acoustic chamber 11 by a nonporous, loss free acoustical conduit 14 having a cross section configuration substantially the same as the end cross section of the acoustic chamber 11. Both condenser microphones (Bruel & Kjaer, type 4133) and piezoelectric microphones (Lafayette No. 994509) have been found suitable for use in this invention. Indeed, the selection of the type of microphone is not critical and many different varieties may be employed It will be appreciated that it is within the purview of this disclosure to incorporate various propagation mode transition devices, not shown, in lieu of the wave energy transmission means shown in the drawing 14 if the selection of a particular type of microphone necessitates a propagation mode transitron.

In consideration of the configuration of the preferred embodiment, it will be recognized that the degree of flow noise rejection reflected in the signal to noise ratio of an acoustic signal received by the flush mounted depends to a large extent on the standing wave ratio inside the acoustic chamber 11 due to the reflection coefiicient of the front end. The signal to noise ratio is optimum if the front end of the acoustic chamber 1 1 is appropriately temiinated to be nonreflective.

An eflectively nonreflective termination is achieved in this invention by internal disposition of the reflective surface 15 which extends from one end of the acoustic chamber 11 to the other.

It has been found that in the relatively low frequency application of the sensor of this invention, the flow impedance characteristic of the sensitive surface 12 and the characteristic acoustic impedance of the microphone connected end cross section of the acoustic chamber 11 should be substantially the same.

It will be appreciated that the sensitive surface 12 may be planar, as shown in FIG. 1, or in the case of a tubular vehicle, may be rounded in cross section to conform with the external surface of the vehicle, if desired. Surface conformance generally serves to minimize pressure fluctuation in the immediate vicinity of the sensitive surface and is feasible from an acoustic chamber consideration in most applications.

In a typical embodiment, as shown in FIG. 1, utilizing the 12 inch by 1.5 inch sensitive surface heretofore described, the rectangular cross section at the microphone connected end of the elongated acoustic chamber would be 1.5 inches in width and 0.196 inches in height. A first, linear, wave energy reflective surface 15 is angularly disposed with respect the sensitive surface 12 to intersect therewith, whereby the opposite end of the elongated acoustic chamber is appropriately terminated.

FIG. 2 shows an array of four sensors 21, 22, 23 and 24 in a typical homing system orientation. The sensors 21, 22, 23 and 24 are electrically connected to respective narrow band pass channels 31, 32, 33 and 34, each comprising a cascade connection of preamplifier, modulator, and band pass filter. The channels 31 and 33 are connected to a phase measuring circuit 41 to provide a first phase difi'erence signal and the channels 32 and 34 are connected to a phase measurement circuit 42 to provide a second phase difierence signal.

Tracking a narrow band filtering of a selected harmonic tone of the target signal is achieved by heterodyne technique. A variable frequency oscillator 51, which may be voltage controlled oscillator, is connected to the modulator in each of the channels 31, 32, 33 and 34 to modulate the sensor output.

In a typical case, a symmetrical voltage controlled oscillator, one which suppresses the carrier frequency of the oscillator, f; at its output, is employed as the modulator. Identical fixed bandwidth band pass filters are employed in each of the channels 31, 32, 33 and 34 and each band pass filter is tuned about a center frequency, 11,, to capture a part of the upper sideband of the modulated signal entering the band pass filter. Thus, a change in frequency, j}, of the modulator effectively enables a scanning of the frequency spectrum of the sensor signal within bandwidth limits of the band pass filters.

In the homing system application, the tracking function is achieved by the addition of a frequency discriminator 52 responsive to the outputs of each of the channels 31, 32, 33 and 34. The output of the frequency discriminator 52 controls the frequency, f of the modulator 51. Thus, if a significant portion of the modulated signal falls outside the pass band of the filters, the output of the channels is reduced in magnitude and the frequency discriminator responds in a conventional feedback manner to shift the modulator frequency, f such that the modulated target signal falls within the pass band of the filters.

The phase measuring circuits 41 and 42 each function in a relatively conventional manner, first clipping each input signal, then obtaining pulses from the axis crossings of the clipped wave train and thereafter generating a rectangular wave train whose ON duration is proportional to the time difference between axis crossings of the two input signals. Thus, the mean value output of each phase measuring circuit is a linear function of the phase delay between pairs of sensors.

While the electronic circuitry of the various elements of the channels 31, 32, 33 and 34 as well as associated modulator, frequency discriminator, and phase measuring means may be conventional, it has been found that phase matching throughout is vital to operation of the system. That is, the phase of each pair of sensors and associated channels must be well matched throughout.

In practice, this phase matched requirement means that in the normal frequency range of the acoustic sensor, 300 1000 cycle/sec, (l) the acoustic chambers in each pair of sensors should have nearly identical physical dimensions and porosity characteristics, as well, including internal reflective structure (2) the microphones in each pair of channels should be phase matched with respect each other and their respective sensors (3) the preamplifiers should be phase matched and (4) the narrow band filters in each channel should be phase matched.

Other elements of the electronic circuitry, such as the amplitude clippers and the modulator 51 usually have a rather wide bandwidth and consequently do not present a phase matching problem.

It will be seen that the dynamic performance of the homing system illustrated in FIG. 2 is controlled primarily by the response characteristic of the band pass filters in each of the channels 31, 32, 33 and 34. Accordingly, it has been found desirable to make the response of the feedback loop within the heterodyne oscillator circuitry faster than the response of the filters. This provides a steady state gain with gain dependent upon the signal to noise ratio (SNR) of the sensor output.

In detemiining the signal to noise ratio (SNR) the flow noise signal, derived from movement of the sensor vehicle, is considered as the main source of noise, at each sensor and the noise is assumed to have the same magnitude, frequency spectrum and bandwidth at each sensor. For a target signal of selected value of db relative to 2 X 10"microbar rms pressure at the location of the sensor, travelling with a velocity of 200 feet/sec, a signal to noise ratio of abut 15 db is readily obtainable, if the signal and the flow noise are measured at a frequency of 1000 cycles/sec in a 28 percent relative bandwidth.

In comparison, utilizing the same target signal in an identical noise environment, a sensor having an identical size and configuration sensitive porous surface and an acoustic chamber of uniform cross section throughout, that is, without the angularly disposed reflective surface indicated at in FIG. 1, would provide a signal to noise ratio of approximately 6 8 db.

It will be appreciated that the acoustic sensor exemplarily depicted herein may be modified in accordance with standard aerodynamic practice as required in selected applications without departure from the purview of this disclosure.

I claim:

1. An acoustic sensor adapted for flush mounting in the surface of a vehicle moving in a selected fluid medium comprising an elongated, substantially wedged shaped acoustic chamber with a first porous surface and a second nonporous wave energy reflective surface angularly disposed with respect said first surface, plus third and fourth substantially triangular side surfaces defining a first end of selected cross section, with the intersection of said first and second surfaces forming the second end of said acoustic chamber; said second end of said acoustic chamber disposed in the surface of said vehicle as the leading end in the direction of travel of said vehicle; said first surface being relatively thin with a multitude of randomly distributed and substantially closely spaced, finite, nondescript channels detemiining a substantially constant flow resistance, F in said fluid medium at relatively low level pressure differential thereacross; said flow resistance, F being substantially equal to the acoustic impedance of said selected cross section of said first end of said acoustic chamber; a pressure sensitive energy transition means having an electrical signal output proportional to a pressure wave input; and means connecting said energy transition means to said first end of said acoustic chamber.

2. An acoustic sensor as defined in claim 1 wherein both said first and second surfaces are planar and the intersection thereof forming said second end of said acoustic chamber is substantially a straight line.

3. An acoustic sensor as defined in claim 1 wherein said first surface is made of sintered stainless steel particles of selected micron size and said first surface has a substantially uniform porosity over substantially the entire surface area thereof. 

1. An acoustic sensor adapted for flush mounting in the surface of a vehicle moving in a selected fluid medium comprising an elongated, substantially wedged shaped acoustic chamber with a first porous surface and a second nonporous wave energy reflective surface angularly disposed with respect said first surface, plus third and fourth substantially triangular side surfaces defining a first end of selected cross section, with the intersection of said first and second surfaces forming the second end of said acoustic chamber; said second end of said acoustic chamber disposed in the surface of said vehicle as the leading end in the direction of travel of said vehicle; said first surface being relatively thin with a multitude of randomly distributed and substantially closely spaced, finite, nondescript channels determining a substantially constant flow resistance, Fx, in said fluid medium at relatively low level pressure differential thereacross; said flow resistance, Fx, being substantially equal to the acoustic impedance of said selected cross section of said first end of said acoustic chamber; a pressure sensitive energy transition means having an electrical signal output proportional to a pressure wave input; and means connecting said energy transition means to said first end of said acoustic chamber.
 2. An acoustic sensor as defined in claim 1 wherein both said first and second surfaces are planar and the intersection thereof forming said second end of said acoustic chamber is substantially a straight line.
 3. An acoustic sensor as defined in claim 1 wherein said first surface is made of sintered stainless steel particles of selected micron size and said first surface has a substantially uniform porosity over substantially the entire surface area thereof. 